This invention relates to a hydraulic control, particularly for an automobile brake system comprising two separate independent circuits for safety reasons, in which each circuit may comprise brake cylinders distributed over the various wheels, the hydraulic control being of the kind in which a master cylinder comprises a first chamber in which pressure is controlled by the driving-in of a master piston under the action of a brake pedal, said first chamber forming part of a first hydraulic closed system comprising a first brake circuit, the master cylinder also having a second pressure chamber forming a part of a second brake circuit, the said second chamber being adapted to be fed by a source of fluid pressure, the said master cylinder being provided with a plunger whose two opposite faces are subjected respectively to the pressures of the two closed systems and forming a valve which cooperates with a seat to define an annular passage of variable section interposed between the second enclosed system on the one hand and a pipe discharging to a reservoir on the other hand, in such a manner that under the action of the pressure applied by the master piston in the first enclosed system for braking purposes the plunger assumes a position of equilibrium in which the passage section is modulated and with the aid of which the pressures in the two enclosed systems are kept proportional (for example equal).
The present invention has as an object a hydraulic control for a braking device, particularly in an automobile vehicle, of the kind indicated above, with the aid of which excellent operating performance is obtained both under normal conditions and in the event of the failure of one of the circuits.
According to the invention a hydraulic control, particularly for a braking device of an automobile vehicle, is characterised in that the plunger is mounted in a part accommodated in the body of the master cylinder. The plunger preferably has a head having a conical annular peripheral bearing surface applied against the seat, and a frontal centre part surrounded by the said conical bearing surface and having a preferably conical ogival shape, of which the half-angle at the apex is slightly greater than the half-angle at the apex of the conical bearing surface.
Due to this device excellent conditions are found for the movement of the leakage flow from the second vessel to the reservoir, with appropriate modulation.
In a preferred embodiment the conical bearing surface has a half-angle at the apex of between 40.degree. and 50.degree., advantageously close to 45.degree., while the ogive has a half-angle at the top which is between 50.degree. and 60.degree., advantageously close to 55.degree..
The head of the plunger preferably extends into a cavity provided in a part engaged in the body of the master cylinder and having passages for communication between the said cavity and the reservoir, which passages are always open whatever the position in which this part is installed.
According to another aspect of the present invention the master piston of the master cylinder which controls the pressure in the first closed system is disposed in the first chamber coaxially with the plunger in such a manner as to permit mechanical intervention in the event of hydraulic failure in the first closed system, the master piston being returned by a first spring and receiving a rod which is adapted to come into contact with the plunger, this rod being coupled to the master piston by a second spring which has no initial stressing and which serves as a simulator of resistance to the depression of the brake pedal.
According to yet another aspect of the invention the end of the plunger which is subjected to the pressure of the first closed system has a section different from that of the seat associated with the second closed system. This makes it possible to obtain an amplification effect or, where necessary, to achieve perfect equalisation of the pressures in the two closed systems.
The master piston is preferably in the form of a hollow sleeve in which is engaged a collar of the rod forming a movable seat for the second spring.
As an alternative the master cylinder may have a bore provided with a shoulder against which the said attached part is applied, while an annular recess is defined, on the first chamber side, between the bore of the master cylinder and the plunger and a seal is inserted into this annular recess with its outer face applied against the bore of the master cylinder and its inner face applied against the plunger. For preference the seal has a generally U-shaped section, with two lips forming respectively the faces applied against the bore of the cylinder and against the plunger and extending towards the first chamber.
An arrangement of this kind has the advantage of not only ensuring economy in construction, because a single seal performs the functions of two, but also of improving conditions of tightness. The pressure of the first chamber acts in fact directly and symmetrically on the two lips of the seal, thus ensuring excellent sealing by the two lips.
According to another characteristic, means are provided for holding the seal in the axial position and consist of a return spring for the master piston.
The master piston preferably cooperates on the one hand permanently with the said return spring and on the other hand, in the event of the failure of the first vessel, after the master piston has been retracted, with a second spring acting between the master piston and the plunger in order to provide modulating resistance simulating a back-pressure.
The source of fluid pressure may be of the permanent flow volumetric type or else of any other type, particularly of the constant pressure type.